18 February, 2026

Plants as sessile organisms





  • Mostly defined by their total inability to move


  • Is this a good thing or a bad thing?


  • Their Darwinian struggle for growth, survival and reproduction in very different arenas has resulted in an extremely wide variety of form and function (Diaz et al 2016)

Themes in vascular plant form

  • Plants extract materials from environment
    • ~18 essential elements
      • soil (most)
      • atmosphere
    • many materials are limited


  • Above- and belowground body plan adapted to extract resources
    • connected via vascular tissues


  • Plants exhibit indeterminate growth
    • “foragers”
    • diversity of sizes & shapes

Diversity in plant form via resource uptake vs support

Plants adapt form to persist in different environments

Plants have 3 tissue systems connecting all plant organs

How do plants create such dynamic structures? Cells!

Epidermal cells: interact with the environment

Outermost layer of cells on a plant, forming a protective barrier that covers the entire plant organ


Aboveground cells = protective barrier, Belowground cells = absorptive surfaces

Parenchyma cells: living cells with metabolic function
















Parenchyma cells in leaves = Mesophyll cells Contain chloroplast and carry out photosynthesis

Collenchyma (flexible) & Sclerenchyma (rigid) for support

Although quite different, both cells types provide mechanical support for plants

Vascular tissues: Xylem composed of water conducting cells


Vascular tissues: Phloem composed of sugar conducting cells


Plant organs are easy! Leaves, stems and roots




Plant cells differentiate and organize into specialized tissues (dermal, ground, vascular) that combine to form functional plant organs*


All 3 tissue types are present in each plant organ


Flowers/Cones are reproductive organs that are highly modified versions of leaves

Dynamic root growth integral for plant function

Roots supply nutrients & water for growth & metabolism


Roots uptake resources through osmosis and diffusion (actively/passively)

Roots can also faciliate resource uptake by pumping H+ into the soil, which protons bind to negatively charged soil particles, displacing nutrient cations (like K+, Ca2+, Mg2+) into the soil

Roots are a member of a living soil community

Indeterminate growth occurs at tips of stems/roots (meristems)

Undifferentiated cells divide until gene expression ‘tells’ them what cells to turn into

Cambium produce secondary growth (out not up) in stems

Stems are mostly vascular tissue, with xylem and phloem forming the main transport system that runs through the plant

Vascular cambium creates new xylem and phloem (tree rings) and cork cambium creates cork (bark)

Leaf structure and function = biomass factory

Leaves exchange gases via stomata = CO2 in, O2 and H2O out


- CO2 enters leaf air spaces and then the chloroplast organelle through a concentration gradient
- Stomata are not always open….WHY???

Leaf gas exchange impacts the global atmopshere

Vascular tissues in leaves are confined to veins


  • Transpiration (leaf water loss from evaporation) is a by‐product of opened stomatal pores

  • H2O transport needs to be sufficient to keep stomata open

  • Veins represents investment in H2O supply

Angiosperms have evolved higher vein densities


  • Helps explain angiosperm dominance
  • Transpiration alters global climate systems! About 10% of water in atmosphere comes from leaves

Leaf economic theory: Return on Investment